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A Unix shell written in C.



This a Unix shell similar to sh, bash and zsh. This implementation is not based on (or exploit) another shell. For example, system(3).

Implementation Details

Example commands

## simple commands:
sleep 5   # must not show the prompt too early
## simple commands, with built-ins:
mkdir t
cd t
/bin/pwd  # must show the new path
exit 42   # terminate with code
## sequences:
echo hello; echo world # must print in this order
exit 0; echo fail  # must not print "fail"
## pipes:
ls | grep t
ls | more    # must not show prompt too early
ls | sleep 5 # must not print anything, then wait
sleep 5 | ls # must show listing then wait
ls /usr/lib | grep net | cut -d. -f1 | sort -u
## redirects:
>dl1 ls /bin; <dl1 wc -l
>dl2 ls /usr/bin; >>dl1 cat dl2 # append
<dl2 wc -l; <dl1 wc -l # show the sum
>dl3 2>&1 find /var/. # errors redirected
## detached commands:
sleep 5 &  # print prompt early
{ sleep 1; echo hello }& echo world; sleep 3 # invert output
## sub-shell:
( exit 0 ) # top shell does *not* terminate
cd /tmp; /bin/pwd; ( cd /bin ); /bin/pwd # "/tmp" twice
## environment variables
set hello=world; env | grep hello # prints "hello=world"
(set top=down); env | grep top # does not print "top=down"

# custom PATH handling
mkdir /tmp/hai; touch /tmp/hai/waa; chmod +x /tmp/hai/waa
set PATH=/tmp/hai; waa # OK
unset PATH; waa # execvp() reports failure

Syntax of built-ins

Built-in: cd <path>
Change the current directory to become the directory specify in the argument. Your shell does not need to support the syntax cd without arguments like Bash does.
Built-in: exit <code>
Terminate the current shell process using the specified numeric code. Your shell does not need to support the syntax exit without arguments like Bash does.
Built-in (advanced): set <var>=<value>
Set the specified environment variable. Your shell does not need to support the syntax set without arguments like Bash does.
Built-in (advanced): unset <var> (optional)
Unset the specified environment variable.

Error handling

This shell might encounter two types of error:


  1. A shell usually supports redirections on all places of a simple command; ls > foo and >foo ls are normally equivalent. However, this shell only supports >foo ls.
  2. Within a ‘pipe’ construction, all parts should be forked, even if they only contain built-in commands. This keeps the implementation easier.
exit 42 # closes the shell
exit 42 | sleep 1  # exit in sub-shell, main shell remains

cd /tmp # changes the directory
cd /tmp | sleep 1  # change directory in sub-shell
                   # main shell does not


  1. install dependencies on Ubuntu using the following command:

     sudo apt install build-essential python python-pexpect libreadline-dev flex valgrind
  2. Use make check to run tests.